Reaction times

Plants can respond slowly or quickly, depending on the stimulus

Tropisms make plants grow differently in response to environmental stimuli such as light, gravity or even a fence.

Plants can also change position and react to their surroundings quickly when they need to. For example, the stomatal openings in leaves that allow gas exchange and water loss can go from closed to fully open within ten minutes. This is controlled by the surrounding guard cells taking up water by osmosis, inflating and pushing the cells apart around the pores. According to some studies, the shape of the guard cells influences the speed of opening, with the dumbbell-shaped cells of grasses permitting faster opening than the kidney-shaped guard cells belonging to other plants. The stomata can be quickly closed as well; the hormone abscisic acid helps in this case.

A striking example of rapid-fire plant movements is the release of pollen from the anthers of the white mulberry tree. These catapult-like movements can be triggered by exposure to dry air. In 2006, scientists used high-speed cameras to capture bent stamens springing straight in less than 25 millionths of a second. The force required to generate the movement comes from build-up of turgor pressure (in which the plasma membrane is pushed against the cell wall) in cells in the stamens.

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